The present invention relates to a method for providing ink jet printing with light-curing ink and a printer for carrying out this method.
An ink jet printer typically comprises a carriage that is movable in reciprocation in a main scanning direction across a recording medium, e.g., a sheet of paper. The carriage has mounted thereon a number of printheads, at least one for each color, so that dots of liquid ink may be expelled onto the surface of the paper, and a stripe or swath of the image is printed during each pass of the carriage. Then, the paper is advanced in a sub-scanning direction, so that the next swath may be printed.
The ink that has been deposited on the paper should be cured, i.e., caused to solidify, relatively quickly, so that the image will not be damaged in subsequent handling or processing steps. Solvent-based inks may be cured by letting the solvent evaporate from the ink. A hot melt ink jet printer uses ink that is solid at room temperature and is melted in the printhead, and curing is achieved by allowing the ink to cool down. In a print process using light-curing ink, the solidification of the ink is induced by irradiating the printed ink dots with light, preferably UV light. This has the advantage that the curing process can be accelerated and the curing time can be finely controlled, thereby controlling the amount in which the printed ink dots flow out on the paper.
However, the light rays used for irradiating the ink dots must be strong enough to penetrate into the volume of the ink. This requirement is particularly challenging when taking into account that the colorants, i.e., the dyes and/or pigment, of some ink may have a low transparency for light rays. Examples of ink with low transparency colorants are black ink, as the black colorant may be highly absorptive for the light rays, and white ink, as the white colorant may be highly reflective for the light rays. Even when the irradiation with light is used only for “pinning” the ink dots, i.e., for solidifying only a certain ink layer at the surface and at the rim of the dot, the light has to penetrate into the ink to some extent. Thus, the power of UV lamps used for curing the ink increases with increasing printing speed and also with decreasing transparency of the ink. As a result, especially the low transparency of black ink and white ink necessitates the use of relatively strong and expensive UV lamps.